This invention relates to a memory system which is suitable for use in combination with a color graphic display or a color printer.
In recent years the memory capacity of IC memory has increased and the cost has been reduced. This has made it possible to provide a raster scanning color graphic display system which has an IC memory of large capacity, which is compact and inexpensive. Such a color graphic display is schematically shown in FIG. 1. The system comprises a control device 1, a function generator 2, an external interface circuit 3, a memory control circuit 4, bus drivers 5, and three memory planes 6, 7 and 8. The memory planes 6, 7 and 8 are used to store data representing picture patterns of the three primary colors, i.e., red, green and blue, respectively.
Let us assume that a host computer (not shown) provides an instruction through interface 3 that a white circle with radius r and center (x.sub.1, y.sub.1) be drawn. The control device 1 receives the data representing r and (x.sub.1, y.sub.1) and then supplies this data to the function generator 2. At the same time, the device 1 instructs the function generator 2 to calculate the coordinates of any point on the circle. The function generator 2 does this calculation and informs the control device 1 of the end of the calculation.
The picture data representing the coordinates of points corresponding to the points on the circle to be drawn are read from the memory plane 6 and supplied to the control device 1. The control device 1 performs a logical operation on the data which represent the coordinates of each point on the circle and the picture data which represent the coordinates of the corresponding point and which have been read from the memory plane 6. The logical operation may be REPLACE operation for drawing a new picture pattern, or SET operation for changing the binary value of a data stored in the memory plane 6 to "1". The result of the logical operation is written into the memory plane 6.
The sequence of operations described in the preceding paragraph are performed on the picture data stored in the other memory planes 7 and 8 also. Hence, some of the picture data stored in each memory plane, which represent the coordinates of the points on the circle to be drawn, are modified. The modified picture data are read from the memory planes 6, 7 and 8 by a display control circuit 9 in synchronism with display timing signals. Thus, they are displayed by a display (not shown), e.g., a CRT, in the form of a white circle.
With the conventional display system described above, it is necessary to perform a logical operation on the data from the function generator and to write the result of the operation into each memory plane. In other words, three similar operations must be effected one after another and the results of these operation must be written into the three memory planes upon completion of the respective logical operations. Hence, the speed at which the whole system operates is inevitably low.